To unlock the potentials of tailoring the microstructure of high-performance metal alloys by variation of manufacturing process parameters, it is necessary to find and describe correlations between resulting microstructural features and the desired mechanical properties. This study aims to investigate and rationalize correlations between mechanical properties and microstructural parameters on the example of the nickel superalloy IN718. Several variants of this material from cast, wrought and multiple laser powder bed fusion (PBF-LB/M) process setups, combined with different heat treatment options have been analyzed regarding the influence on various microstructural characteristics, such as grain sizes, grain boundary angles, twining, γ- and δ-precipitations. This approach was chosen to generate a wide range of microstructural and mechanical characteristics and then link the found correlations to macroscopic properties relevant in component design, such as crack growth thresholds.
The microstructure of the manufactured specimens was analysed in depth utilizing a broad variety of characterization measures like energy dispersive X-ray spectroscopy (EDX),electron backscatter diffraction (EBSD) as well as secondary electron and backscatter electron contrast in a scanning electron microscope. The precipitate structure was analyzed quantitatively using the image processing software “ImageJ”. The particle size distribution, shape factors and percentage of area of the incoherent Ni3Nb-δ-phase is of special interest, since its presence is highly dependent on the thermal conditions during build processes and heat treatments, and it is discussed as a root cause for the embrittlement of this alloy in the temperature range between 600 °C and 650 °C.
To determine macroscopic mechanical properties, standard tensile tests at room and high temperature were carried out. Hardness measurements were carried out by nanoindentation. Application relevant properties such as crack growth thresholds and creep life from accompanying test campaigns were included in the discussion of the analysis results.
The determined microstructural parameters were then correlated with mechanical parameters, using the Pearson sample correlation coefficient as base metric. A comparison with the literature showed that the calculated correlation coefficients not only reflect expected basic correlations such as the Hall-Petch relationship. The influence of the δ precipitates described in the literature is also reflected in the correlation analysis. Also, material specific influences e.g., of the δ-phase on plastic deformability were revealed. Furthermore, effects of the proportion of small angle and twin grain boundaries on some mechanical properties could be determined and are discussed in this paper. Especially the observed influence on the long crack growth threshold value is of interest in the scope of material design for certain component loading conditions.